Hi Hannes,
Having replaced a lot of iPhone screens thanks to young grandchildren who had their own phones, I'd say it's most likely the digitizer connector isn't seated fully. Those FPC connectors can sometimes be tricky to get in exactly right; they can look like they're seated when they're not actually.
So I'd say your next step would be to open it back up and reseat the digitizer connector. It's the one on the right; here's a picture showing the spudger prying it loose.
[image|3544611]
So follow the screen replacement guide up to Step 23 and unplug both connectors. Examine them with a magnifying glass and a bright light on both sides for any bent, missing or broken pins; bent pins can sometimes be straightened with fine tip tweezers. Once you're satisfied everything is good, clean all the connectors with 90% or higher concentration isopropyl (rubbing) alcohol, give them a moment to dry, then carefully plug everything back in.
I have the best luck with those connectors by positioning the cable over the socket then lightly place your fingertip on the connector. Press gently while wiggling your finger slightly till you feel the connector drop slightly into the socket. Once it does, you can press more firmly and you should feel it pop in to where it's seated. Visually check that it's completely flat and both are the same height, then put the cover on.
Finally, test the touchscreen before you seal the phone back up; you may need to unplug it again in order to redo the adhesive if needed.
If you still have no joy, remember that iFixit screens have a lifetime warranty so your next step would be to contact the iFixit Store personnel and explain the issue. In my experience they are more than willing to work with you to make things right. Best way I know of to get hold of them is by emailing them at support@ifixit.com.
Good luck, and let us know how it all works out!